Originally being particularly high end (read.. eye wateringly expensive) test gear for use in vibration and acoustical analysis almost all of the units have had to be modified for studio use mainly to tame their output stages because up to 90V DC at the output terminals is not exactly mixer friendly.

The all tube 2107 BPF with its continuously swept tuning with up to 50dB slopes in max regeneration!..

Currently working on a gorgeous pair of model 2112 all tube band pass filters with something like 120dB / Octave slopes - one of which has been a particularly arduous to return to life...

And its all hand cranked goodness as there is no way in heck this lot could be voltage controlled without the use of a large servo motor...

Last edited by HideawayStudio on Sat May 05, 2018 9:57 am, edited 1 time in total.

Currently trying to get some of the scruffier black outer casings stripped and powder coated but the original paint is proving to be so robust even sand blasting is proving slow going.

In testament to robust design, despite being 50 years old, to date 14 out of the 15 units I've worked on this year have been returned to life with one sadly being robbed of several custom resonator inductors to return a more interesting model to life.

Most of these units are 50 years old and arrive absolutely filthy inside and out. Each unit is meticulously cleaned in inside and out and visually inspected. The transformer winding to the 410 Volt HT rail is temporarily disconnected and the unit slowly run up to test the heater rails including the regulated DC heater rails to the tubes on the front end of the input and output amplifiers (only very expensive designs had regulated DC heater rails in those days!).

A variable high voltage and current limited bench supply is wired into the B+ rail and the HT is slowly brought up to around 70 volts to check for excessive loading in relative safety. It is quite surprising how many tube designs will work at this sort of voltage as long as the heaters are running at their proper temperature. A very quick functionality test is carried out. If it looks promising the B+ voltage is slowly increased to 150 volts and the unit left running for several hours whilst monitoring for temperature build up in the electrolytic caps.

Usually a plethora of grid decoupling caps, burnt out cathode follower resistors and the occasional excessively leaky HT smoothing cap are replaced along with fresh pilot lamps in the meter and tuning dials.

The bizarre gearbox on the rear of the model 2107 is part of an elaborate mechanism to auto-switch between adjacent frequency ranges when the huge tuning dial is turned to either extreme meaning that you can sweep the entire 20Hz to 20KHz audio spectrum in a handful of turns of the dial.

The active tube regeneration (all tube!) allows slopes from around 20dB/oct to nearly 50dB/octave and with the studio mods everything can go into glorious saturation if required.

This reminds me of a conversation I once had with Oskar Sala. He pointed out that the 500V required for feeding the valves more often than once gave him a shock when getting too close to one of the voltage rails. The designers of the semiconductor-based Trautonium were in utter disbelief when they saw the wiring harness inside Sala's early Mixturtrautonium -- electrical safety? Yes... just don't touch.

Did I mention the 25A that were required as well?

Stephen

"Like the light from distant stars, Stephen Parsick's music has existed for some time, but is only now reaching us on Earth." Chuck van Zyl

I have just completed restoring two late 1960s Bruel & Kjaer all tube Band Pass Filter sets modified for studio use being tested as a stereo pair for the first time shortly after restoration at Hideaway Studio.

I am now (somewhat nervously!) trying to get some hours on them during soak testing.

This is the first time I've attempted to run up two of these unique filters as a stereo pair:

First you hear the sound sent through the filters in non-selective "Linear" mode and then the selective band pass filtering is enabled.

The source instrument in this case is a little Alesis Micron playing a single sustained chord whilst the two filters are set to different tuning positions.

They sound particularly beautiful when each unit is set to adjacent 1/3oct bands.

Unlike the 2107s these units filter in steps using a large bank of high Q L/C resonators (a coil and a cap in parallel as a resonant tank). Although each step is at 1/3rd octave spacing, at the crest of each L/C resonator band the slopes fall away at over 100dB/octave which is very considerably more than most analog studio filters. This highly selective filtering results in an almost haunting quality. Filtering white noise through these sounds quite eerie too.

Is anyone here who would like to buy such a device:Bruel & Kjaer 2112 Frequency SpectrometerPlease apologize if this is the wrong place.I inhereted the 2112 from my father and try to find someone who loves to play with it.Location: close to StuttgartBrukja